Hypoxia and hypercapnia affect contractile and histological properties of rat diaphragm and hind limb muscles.

The effects of hypoxia and hypercapnia on contractile and histological properties of the diaphragm and skeletal muscles of the hind limb were examined. Eight-week-old male Sprague-Dawley rats ( [Formula: see text] ) were kept in hypobaric hypoxic ( [Formula: see text] ) or hypercapnic ( [Formula: see text] ) chambers for 6 weeks, and compared with the control rats (room air, [Formula: see text] ). Contractile properties were evaluated with twitch kinetics, force-frequency curve and fatigue tolerance. After the experiments on contractile activities, muscles were fixed for histological examination with ATPase staining. It was demonstrated that peak twitch tension of diaphragm decreased with no significant histological changes under hypoxic conditions while significant contractile and histological changes were observed under hypercapnic conditions. Skeletal muscles of the hind limbs were affected also under hypoxic and hypercapnic conditions but the profiles of the changes in contraction and histology were different from those of the diaphragm. These results suggest that hypoxia and hypercapnia affect differently on contractile and histological properties of respiratory and hind limb muscles. Furthermore, when we consider the conditions involved in chronic obstructive respiratory disease (COPD; both hypoxia and hypercapnia are deeply involved), our results indicate that COPD should be regarded as a systemic disorder rather than a respiratory disease.